This invention relates generally to preheating and/or curing of furnace linings, and especially the refractory lining of the channel in an induction furnace. Such a furnace is often used either as a holding furnace to maintain the temperature of a quantity of molten metal such as steel, or as a melting furnace for melting down ore or scrap.
A channel type induction furnace commonly includes a hearth or chamber having walls which are lined with refractory material to withstand the high temperature of the molten metal contained therein. In such a furnace, one or more generally U-shaped pr W-shaped channels communicate in open flow relationship with the furnace chamber to define a circulatory passage from the furnace chamber through the channel and back to the furnace chamber. Intermediate the open ends thereof, the channel is encompassed by electrically conductive media such as copper wire windings to form one or more inductors. When the windings are energized, an inductive field is established which is effective to circulate molten metal through the channel and to inductively heat the molten metal as it passes therethrough. Such circulation and heating of the melt permits precise control of the metal temperature until such time as further processing thereof is undertaken.
Since the introduction of channel type induction furnaces many years ago, practitioners of the art have sought improved means for heating and curing the furnace channel. The refractory material used in a newly replaced channel lining must be properly heat cured before use. Thereafter, with each additional heatup of the furnace the channel lining must be properly preheated to drive out absorbed moisture which could otherwise cause the channel lining to crack or spall upon sudden exposure to the extreme temperature of the molten metal. The extreme abrasion of molten metal flow to which the channel lining is continuously exposed during a furnace campaign also dictates carefully controlled curing of a new channel lining prior to first use and preheating for each subsequent use.
To heat cure a furnace channel, temperatures in the range of 2400.degree. F. to 2500.degree. F. are generally required; however, as the channel usually is only about 11/2 to 4 inches in interior diameter and 5 to 6 feet in length, and additionally since the nearest access thereto is commonly at the top of the furnace which is usually at least 4 to 11 feet away from the channel entrance, it has often been impossible to obtain uniform channel temperatures above 2100.degree. F. during channel preheating and/or curing.
There are only two prior approaches to channel heat curing are few. One involves use of an elongated gas pipe burner of 11/2 to 2 inches in diameter. The burner pipe is inserted through an access opening in the top or lid of the furnace such that the combustion zone of the burner remains outside of the furnace. This "lazy flame" burner projects the hot combustion products through the length of the burner pipe and toward the furnace channel for heating thereof. Initially, the burner is fired with the highest air volume flow rate that is possible without causing a flameout. The high volume air flow is necessary to help push the hot combustion products through the channel. This is conducive to good heat flow and also helps somewhat to keep the flame temperature low during initial stages of the cure.
Several inherient problems with the use of such burners have been observed. For example, in spite of the high volume air flow, only relative low flame velocity is realistically attainable, a velocity which is insufficient to uniformly distribute the heat of combustion throughout the length of the furnace channel. Additionally, this type of burner has a very poor turndown (i.e. the burner exhibits a low air-to-fuel gas volume ratio and a narrow air to gas volume ratio operating range). As a result, the burner tends to run very hot even at relatively low gas flow rates. The same limitation requires air flow to be reduced as fuel gas flow is increased during progressive heating. Thus, any cooling effect of the air flow on the burner pipe walls is reduced and the remaining burner pipe cooling capacity is more than offset by the heating effect of the furnace atmosphere.
As a result, this burner type is inadequate to the task at both extremes of operation. The initial hot operation has a tendency to thermally shock the channel lining while the inability to efficiently project combustion heat into the channel at higher operating temperatures and to cool the burner pipe often results in premature burner failure and/or inability to generate the desired maximum cure temperature. Burner failure often is manifested as premature slumping as the burner pipe sidewalls become non-self supporting and sag at elevated temperatures of 1800.degree.-2200.degree. F., which is less than the desired maximum cure temperature.
A second known approach to channel preheating or curing utilizes a so-called ejector system in which a venturi device is placed over one end of an elongated aspirator tube externally of the furnace. The opposite end of the tube is placed snugly over the channel entrance and the joint therebetween is sealed. Compressed air supplied to the system is directed across the exterior end of the tube to draw furnace atmosphere through the channel and the tube, and thereby heat the channel. This approach has been unsatisfactory in that the material use to seal the aspiratory tube at the channel entrance may be dislodged when the tube is removed, and the sealing material may fall into the channel and form a blockage. Additionally, this approach does not benefit from large volume flow of ambient temperature air in the aspirator tube throughout the curing process and the tube thus is quickly heated to destructive temperatures by the furnace atmosphere passing therethrough. Another limitation of this approach is insufficient temperature control at elevated temperatures. Specifically, as the venturi effect is increased, an increased draft is generated in the channel and the aspirator tube. Although reasonably good temperature control is realized by this approach in lower temperature ranges, the known ejector systems generally have been of insufficient capacity to create sufficient flow to hold the channel temperature close to the furnace atmosphere temperature. As a result, channel temperature tends to lag behind furnace chamber temperature.
With either of the above prior approaches, the problem of premature tube failure could of course be solved by fabricating the burner or aspirator tube from specialty materials which will tolerate the high temperatures without failure. However, such tube structures would be vastly more expensive than those fabricated from more common materials, and the service life thereof would have to be extended enormously over the expected life of more conventional materials to realize any true ecomony.
Among the prior art patents known to applicant are the following. U.S. Pat. No. 4,008,993 discloses the preheating of a furnace channel by means of a circulation generating article and is based on the venturi method discussed above. U.S. Pat. No. 2,655,550 teaches a channel furnace for melting metal. One manufacturer of burners for furnace preheating is North American Manufacturing Company.